1. Field of the Invention
This invention relates to a control system and an EGR failure determination system for an internal combustion engine that has an internal EGR amount and an external EGR amount changed by an internal EGR device and an external EGR device, respectively, the control system controlling the internal EGR amount, the external EGR amount, and so forth, and the EGR failure determination system determining failures of the EGR devices.
2. Description of the Related Art
Conventionally, a control system for an internal combustion engine including an EGR failure determination system have been disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-360548. This internal combustion engine is provided with a variable intake cam phase mechanism and an exhaust recirculation mechanism. When the variable intake cam phase mechanism varies the phase of an intake valve relative to a crankshaft of the engine (hereinafter referred to as “the cam phase”), the valve overlap is changed to change the amount of combustion gases remaining in a cylinder of the engine (hereinafter referred to as “the internal EGR amount”). Further, the exhaust recirculation mechanism includes an EGR passage extending between an exhaust passage and an intake passage, and an EGR control valve for opening and closing the EGR passage. In the exhaust recirculation mechanism, by changing the degree of opening of the EGR control valve (hereinafter referred to as “the EGR opening”), the amount of gases recirculated from the exhaust passage to the intake passage (hereinafter referred to as “the external EGR amount”) is changed.
The above EGR failure determination system includes an EGR control system. The EGR control system controls sets a target cam phase as a target of the cam phase and a target EGR opening as a target of the EGR opening are set according to operating conditions of the engine, and controls the internal EGR amount by the variable intake cam phase mechanism such that the cam phase becomes equal to the target cam phase, and the external EGR amount by the EGR control valve such that the EGR opening becomes equal to the target EGR opening.
Further, the EGR failure determination system compares the absolute value of the difference between the EGR opening and the target EGR opening with a predetermined value, and determines that a failure has occurred in which the EGR control valve is stuck at an open state, when the absolute value of the difference is not smaller than the predetermined value.
Furthermore, when the EGR failure determination system has determined that the EGR control valve is faulty, the EGR control system sets the target cam phase to a value of 0, whereby the cam phase is controlled to a value for minimizing the valve overlap by the variable intake cam phase mechanism. That is, the internal EGR amount is controlled such that it becomes minimum.
The above-described conventional control system carries out failure determination only of a failure in which the EGR control valve is stuck at an open state, that is, a failure in which the external EGR amount is too large, and when such a failure has occurred, the internal EGR amount is only controlled such that it becomes minimum, by the variable intake cam phase mechanism. Therefore, when a failure has occurred in which the external EGR amount decreases to be too small, the total EGR amount becomes so low to make knocking liable to occur, resulting in deteriorated drivability. This problem can become conspicuous particularly during high-load operation of the engine.
According to the above-described conventional EGR failure determination system, the failure of the EGR control valve is determined by comparing the absolute value of the difference between the EGR opening and the target EGR opening with the predetermined value, and hence when the EGR rate is small and the external EGR amount is small, the accuracy of the determination is lowered, which can bring about an error in the determination. That is, the EGR failure determination system can carry out failure determination with accuracy only in a region where the external EGR amount is large, which lowers the marketability of the system.
Further, no failure determination is carried out of the variable intake cam phase mechanism, and therefore when the variable intake cam phase mechanism is faulty, the internal EGR amount is controlled to an improper value by the EGR control system, which results in degraded combustion of the engine. Furthermore, when the EGR failure determination system determines that the EGR control valve is faulty, the EGR control system controls the internal EGR amount by the variable intake cam phase mechanism such that the internal EGR amount becomes minimum, so that knocking is liable to occur during high-load operation of the engine.